I. The Existing Long Term Evolution (LTE) Frame Structures (in the Rel-8/9/10/11/12/13)
The Frame Structure Type 1 (FS1) is applicable to the existing LTE Frequency Division Duplex (FDD) system. In the FDD system, there are different carrier frequencies in, and the same frame structure for, uplink and downlink transmission. FIG. 1 illustrates a schematic structural diagram of the FS1, where a radio frame with the length of 10 ms over each carrier includes ten 1 ms sub-frames, and each sub-frame includes two slots with the length of 0.5 ms. A Transmission Time Interval (TTI) for transmitting uplink and downlink data is 1 ms.
The Frame Structure Type 2 (FS2) is applicable to the existing LTE Time Division Duplex (TDD) system. In the TDD system, there are different sub-frames or slots, at the same frequency, for uplink and downlink transmission. FIG. 2 illustrates a schematic structural diagram of the FS2. In the FS2, each radio frame with the length of 10 ms includes two half-frames with the length of 5 ms, and each half-frame includes five sub-frames with the length of 1 ms. The sub-frames in the FS2 are categorized into downlink sub-frames, uplink sub-frames, and special sub-frames, where each special sub-frame includes three components of a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS), where a downlink pilot, downlink traffic data, and downlink control signaling can be transmitted in the DwPTS; no signal is transmitted in the GP; and only a random access signal and a Sounding Reference Symbol (SRS) can be transmitted, but neither uplink traffic data nor uplink control information can be transmitted, in the UpPTS. Each half-frame includes at least one downlink sub-frame, at least one uplink sub-frame, and at most one special sub-frame. Table 1 depicts seven uplink-downlink sub-frame configurations supported in the FS2.
TABLE 1Uplink-downlink configurationsUplink-Downlink-downlinkto-Uplinkconfig-Switch-pointSub-frame numberurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
II. The DMRS Design of a PUSCH in the Existing LTE System
FIG. 3a and FIG. 3b illustrate structural diagrams of data and pilots (i.e., reference signals, or Demodulation Reference Signals (DMRS's) for demodulating the data), of an LTE Physical Uplink Shared Channel (PUSCH) in a sub-frame. As illustrated in FIG. 3a, with a normal Cyclic Prefix (CP), the pilots are transmitted in the fourth symbol in each slot of each sub-frame, and the data are transmitted in the other symbols than the fourth symbol. As illustrated in FIG. 3b, with an extended CP, the pilots are transmitted in the third symbol in each slot of each sub-frame, and the data are transmitted in the other symbols than the third symbol. An uplink pilot is a UE-specific pilot generated according to the size of a real bandwidth scheduled by the PUSCH. Transmit power of a DMRS is the same as transmit power of data. In order to support the uplink Multi User-Multi Input Multi Output (MU-MIMO) mode, the same pilot-based sequence can be shifted cyclically using each column of pilots to orthogonally transmit pilots of a plurality of UEs sharing the same resource, so that a receiver can distinguish pilot information of the different UEs from each other by shifting the pilot information cyclically.
As there is a changing demand for mobile communication traffic, the traffic is required to be more and more real-time. One of working solutions to shortening a delay is to reduce the length of a TTI.
In the LTE system, the existing channel transmission is generally defined in a sub-frame, and when a PUSCH is transmitted in a shorter TTI than 1 ms, the DMRS structure designed for a 1 ms sub-frame in the LTE system can be reused, and DMRS's of short data transmission in a sub-frame can be transmitted at the same time position, where the short data transmission is defined as data transmission with a smaller length of time than 1 ms. However there is separate scheduling information of a plurality of PUSCHs, and scheduled bandwidths of the respective scheduling information may only overlap partially with each other; and in order to enable the DMRS's of the different short data transmission to be distinguished from each other in the same symbol, pilot sequences of the respective short data transmission can be transmitted in the same resource region in a frequency division multiplexing mode with a comb-like pattern so that uplink data can be transmitted and demodulated correctly. In this way, a plurality of UEs can share the DMRS resources, but there has been absent a definite solution to determining transmit power of a DMRS.